1. Field of the Invention
The present invention relates to the methods used for the measurement of aerosol particles. In particular, the present invention relates to a method that intermittently or continuously changes the aerodynamic cut at the inlet of an optical sensor and thus records the measured concentrations of airborne particles relative to specific aerodynamic particle sizes.
2. Description of the Related Art
The motion in air of airborne particles and their deposition onto surfaces depends primarily on their particle size. Density, shape, and surface characteristics of the particles also influence their airborne behavior and are usually combined with the physical particle size through the "aerodynamic particle diameter". A common method for determining the aerosol size distribution is to collect the aerosol particles on a filter and to subsequently size them under an optical or electron microscope. However, this method only yields the physical size, not the aerodynamic particle size.
A common method for dynamically measuring the concentration and size distribution of particles in the airborne state is by optical single particle size spectrometry. In this method, one particle at a time is passed through an illuminated view volume, and the light scattered by each particle is recorded as to its magnitude. An optical single particle counter using this method is usually calibrated with spherical, monodisperse test particles of known particle density and optical characteristics, such as polystyrene latex (PSL) spheres. However, most airborne particles have their own light scattering and absorption characteristics, so that the "optical particle diameter" measured by this method generally does not correspond to the "aerodynamic particle diameter".
When using the optical single particle counting method, the device embodying this method can be aerodynamically calibrated by placing an impaction stage at its inlet. The "aerodynamic cut" of this impaction stage can be determined by theory or experiment. Thus, one optical particle size of the optical single particle counter can be related to its equivalent aerodynamic particle size. In order to operate the optical single particle counter in the field, the impaction stage is removed from the inlet. If an optical single particle counter is to be calibrated over a wide particle size range, several impaction stages with different aerodynamic cut sizes need to be successively attached to the optical single particle counter's inlet. This is very time consuming.
A simpler dynamic optical particle sensing method is "aerosol photometry". In an aerosol photometer using this method, the optical view volume is larger than in an optical single particle counter, thus accommodating a cloud of particles. However, the output from such a device depends not only on the optical characteristics of the particles, but also on the size distribution of the particles, because the light scattered by each particle depends on its refractive index and size.
In the present invention, the aerodynamic cut is continuously or intermittently varied at the inlet of an optical sensor. This permits the dynamic calibration and determination of the aerodynamic particle size distribution by use of a relatively inexpensive optical device, such as an optical single particle counter or an aerosol photometer.
Another dynamic, but more complex and expensive method for determining the aerodynamic particle size distribution in situ is by accelerating the aerosol particles in a nozzle and then measuring the "time of flight" of each particle between two points. In a nozzle or other acceleration field, aerodynamic drag accelerates large particles to a lesser extent than small particles. Thus, in an acceleration field, the time of flight between two points is longer for large particles than for small particles. The difference is caused by the difference in aerodynamic drag. Thus, this method determines the particle size distribution relative to the aerodynamic particle size. In the following U.S. patents, the time of flight is measured and related to aerodynamic particle size through different techniques and geometric arrangements: U.S. Pat. Nos. 4,633,714; 5,296,910; 5,561,515; 5,679,907; and 5,701,012.